The .223 Remington (.223 Rem) is a cartridge with almost the same external dimensions as the 5.56×45mm NATOmilitary cartridge. The name is commonly pronounced either two-two-three or two-twenty-three Remington. It is loaded with a 0.224-inch (5.56 mm) diameter jacketed bullet, with weights ranging from 40 to 90 grains (2.6 to 5.8 g), though the most common loading by far is 55 grains (3.6 g).[2] While the external case dimensions are very similar, the .223 Remington and 5.56×45mm differ in both maximum pressure and chamber shape. The maximum and mean pressures for some varieties of the 5.56 (different cartridge designations have different standards) exceed the SAAMI maximum for the .223 Remington, and the methods for measuring pressures differ between NATO and SAAMI.[3] The 5.56 chamber specification has also changed since its adoption, as the current military loading (NATO SS-109 or US M855) uses longer, heavier bullets than the original loading. This has resulted in a lengthening of the throat in the 5.56 chamber. Thus, while .223 Remington ammunition can be safely fired in a rifle chambered for 5.56×45mm NATO, firing 5.56 ammunition in a .223 Remington chamber may produce pressures in excess of even the 5.56 specifications due to the shorter throat.[4]

During the late 1950s, ArmaLite and other U.S. firearm designers started their individual Small Caliber/High Velocity (SCHV) rifle experiments using the commercial .222 Remington cartridge. When it became clear that there was not enough case capacity to meet U.S. Continental Army Command's (CONARC) velocity and penetration requirements, ArmaLite contacted Remington to create a similar cartridge with a longer case body and shorter neck. This became the .222 Special. At the same time, Springfield Armory's Earle Harvey had Remington create an even longer cartridge case then known as the .224 Springfield. Springfield was forced to drop out of the CONARC competition, and thus the .224 Springfield was released in 1958 as a commercial sporting cartridge known as the .222 Remington Magnum. To prevent confusion among all of the competing .222 cartridge designations, the .222 Special was renamed the .223 Remington in 1959. In the spring of 1962 Remington submited the specifications of the .223 Remington to the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI). With the U.S. military adoption of the M16 assault rifle in 1963, the .223 Remington in a slightly derived form was standardized as the 5.56×45mm NATO. As a commercial sporting cartridge the .223 Remington was introduced in 1964.[5][6]

The .223 Remington is a cartridge that is ballistically in between the .222 Remington, and the .222 Remington Magnum. The popularity of .223 Remington is so great that in the United States it has virtually eliminated the chambering of new firearms in .222 Remington and .222 Remington Magnum which includes both semi-automatic and bolt (manual) action firearms. (Outside the USA and in particularly parts of Europe where military chamberings are restricted, the .222 Remington remains popular, filling the same market niche.)

According to the official C.I.P. (Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives) rulings the .223 Remington can handle up to 430.00 MPa (62,366 psi) Pmax piezo pressure. In C.I.P. regulated countries every rifle cartridge combo has to be proofed at 125% of this maximum C.I.P. pressure to certify for sale to consumers.[9] This means that .223 Remington chambered arms in C.I.P. regulated countries are currently (2016) proof tested at 537.50 MPa (77,958 psi) PE piezo pressure. This is equal to the NATO maximum service pressure guideline for the 5.56×45mm NATO cartridge.

The SAAMI pressure limit for the .223 Remington is set at 379.212 MPa (55,000 psi), piezo pressure.[10][11]

The rifling in the .223 Rem chambered rifles and pistols varies primarily due to the types of bullets that will be used. The original .223 Rem in the AR-15 had a 1 in 14" (1:14) twist rate to the rifling. This would be sufficient only for the lightest bullets at the highest velocities. Since the Military decided to use a 55 grain fmjbt bullet at introduction of the M16 the 1:14 results in evidence of bullets "keyholing" when hitting paper targets. The rifling was immediately changed to 1:12.[12]

In 1972 Nato adopted the FN Herstal version of .223 Rem with the designation of 5.56 x 45 mm Nato as type SS109. US military 55 grain .223 Rem loaded a 55 grain FMJBT bullet, but NATO 5.56 x 45mm's lightest bullet was 62 grains in the SS109 loading (US number M855).

The .223 Remington is one of the most common rifle cartridges in use in the United States, being widely used in two types of rifles: (1) varmint rifles, most of which are bolt action and commonly have 1-in-12 rifling twist suitable for bullets between 38 to 90 grains (2.5 to 5.8 g), and (2) semi-automatic rifles such as the AR-15 and the Ruger Mini-14, which are commonly found to have twist rates of 1-in-7, 1-in-9, or 1-in-8. (Most modern AR-15s use 1-in-9 which is suitable for bullets up to 69 grains or 4.5 grams or 1-in-7 which is suitable for slightly heavier bullets, but older M16's used 1-in-12 twist rates, making them suitable for use with bullets of 55 grains or 3.6 grams.) The semi-automatic rifle category is often used by law enforcement, for home defense, and for varmint hunting. Among the many popular modern centerfire rifle cartridges, .223 Remington ammunition is among the least expensive and is often used by a wide range of target shooters, particularly in the "service rifle" category or 3 gun matches. The .223 is also used in survival rifles.

.223 remington 1 in 12 twist is not a good for bullets 60 gr & over.

The Sturm, Ruger & Co. AR-556 has rifling at 1:8. Their Mini-14 Target rifle has a rate of 1:9 for example in semi-automatic sporting rifles. Ruger's American Rifle Bolt Action is also in 1:8.[13] Smith and Wesson in their M&P15 also uses 1:7.[14]

The .223 Remington and 5.56×45mm NATO cartridges and chamberings are similar but not identical.[15] While the cartridges are identical other than powder load, bullet weight, chamber pressure and the chamber leade (throating in the USA), i.e. distance from the projectile while seated in the case to the rifling is typically shorter in .223 Remington commercial chambers. Because of this, a cartridge loaded to generate 5.56×45mm NATO pressures in a 5.56×45mm NATO chamber may develop pressures that exceed SAAMI limits for .223 Remington when fired from a short-leade .223 Remington chamber. This is a heavily debated issue among shooters and reloaders, but there has been very little actual research done to determine the actual ramifications of firing 5.56 NATO in rifles chambered for .223.[16]

The throating issue exists because in the USA it has been traditional to have short chambers so that the bullet is being engraved at the moment of insertion. European practice has more of a forcing cone construction which can, by itself, allow significantly higher chamber pressure. All of Sig-Sauer, handguns (for example) have European throating and all are certified to fire +P ammunition. Short throating and unnoticed bullet setback can easily increase chamber pressures by more than 10,000 psi.

The result is that 5.56×45mm NATO ammunition is not as accurate as .223 Remington in many of the AR type rifles extant even with the same bullet weight. A solution to the problem has been developed by Bill Wylde and it bears his name. .223 Wylde is not a cartridge, it is a barrel chamber specification - with the external dimensions and lead angle as found in the military 5.56×45mm NATO cartridge and the 0.2240 inch freebore diameter as found in the civilian SAAMI .223 Remington cartridge - that was designed to increase the accuracy of 5.56×45mm NATO ammunition to that of .223 Remington.[17] A more significant factor is that many early AR-15 rifles were rifled at 1 in 12" twist rates. When FN developed 5.56×45mm NATO SS109 ammunition they used heavier and much longer bullets. Where AR-15 rifles firing .223 Remington with 1 in 12" barrels are quite accurate, that would not be the case if they are used with longer and heavier 62 to 77 grain bullets used in NATO loads or the new commercially available 90 grain bullets. Most .223 Wylde barrels are available with 1 in 6.5" to 1 in 9" twist rate rifling. The twist rate alone is critical for accuracy.

A quick look shows that most US producers of AR-15 rifles are chambering them in 5.56×45mm NATO combined with 1 in 8" or 1 in 9" rifling which will stabilize both most of the .223 Remington and the basic 5.56×45mm NATO cartridges. AR builders who intend to shoot 90 grain bullets will probably gravitate to a 1 in 6.5" twist rate rifling.

Firearms sold in most countries are required to pass certain safety tests, particularly a proof test consisting of firing a special high pressure round (proof load) which far exceeds the European C.I.P.[18] or U.S. SAAMI[19] pressure maximum for the round (see internal ballistics).

The dimensional specifications of 5.56 NATO and .223 Remington commercial brass cases are identical. The cases tend to have similar case capacity when measured, with variations chiefly due to brand, not 5.56 vs .223 designation. The result of this is that there is no such thing as "5.56 brass" or ".223 brass", the differences in the cartridges lie in pressure ratings and in chamber leade length, not in the shape or thickness of the brass.[20][21]

C.I.P. defines the maximum service and proof test pressures of the .223 Remington cartridge equal to the 5.56×45mm NATO, at 430 MPa (62,366 psi). This differs from the SAAMI maximum pressure specification for .223 Remington of 380 MPa (55,114 psi), due to CIP test protocols measuring pressure using a drilled case, rather than an intact case with a conformal piston, along with other differences.[22] NATO uses NATO EPVAT pressure test protocols for their small arms ammunition specifications.

Because of these differences in methodology, the C.I.P. Pmax pressure of 430 MPa (62,366 psi) is the same as a SAAMI pressure of 380 MPa (55,114 psi), which is reflected in US Military specifications for 5.56×45mm NATO, which call for a mean maximum pressure of 55,000 PSI (when measured using a protocol similar to SAAMI).[23][24]

These pressures are generated and measured using a chamber cut to 5.56×45mm NATO specifications, including the longer leade. Firing 5.56×45mm NATO from a chamber with a shorter .223 Remington leade can generate pressures in excess of SAAMI maximums.

The 5.56×45mm NATO chambering, known as a NATO or mil-spec chamber, has a longer leade (free bore), which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 Remington chambering, known as SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber[25] or the ArmaLite chamber, which are designed to handle both 5.56×45mm NATO and .223 Remington equally well. The dimensions and leade of the .223 Remington minimum C.I.P. chamber also differ from the 5.56×45mm NATO chamber specification.

Using commercial .223 Remington cartridges in a 5.56×45mm NATO chambered rifle should work reliably, but until recently, it was believed this was less accurate than when fired from a .223 Remington chambered gun due to the longer leade.[26] Although that may have been true in the early 1960s when the two rounds were developed, recent testing has shown that with today's ammunition, rifles chambered in 5.56 can also fire .223 ammunition every bit as accurately as rifles chambered in .223 Remington, and the 5.56 chamber has the additional advantage of being able to safely fire both calibers.[27] Using 5.56×45mm NATO mil-spec cartridges (such as the M855) in a .223 Remington chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and SAAMI recommends against the practice.[28][29] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56×45mm NATO, such as many commercial AR-15 variants and the Ruger Mini-14 (marked ".223 cal"), but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56×45mm NATO ammunition.[30]

It should also be noted that the upper receiver (to which the barrel with its chamber are attached) and the lower receiver are entirely separate parts in AR-15 style rifles. If the lower receiver has either .223 or 5.56 stamped on it, it does not guarantee the upper assembly is rated for the same caliber, because the upper and the lower receiver in the same rifle can, and frequently do, come from different manufacturers – particularly with rifles sold to civilians or second-hand rifles. On the other hand, the lower receiver is not subject to the majority of the stresses of firing, so the construction of it is significantly less important compared to the upper receiver.

In more practical terms, as of 2010 most AR-15 parts suppliers engineer their complete upper assemblies (not to be confused with stripped uppers where the barrel is not included) to support both chamberings in order to satisfy market demand and prevent any potential problems.[citation needed]

Barrel length helps determine a specific cartridge's muzzle velocity. A longer barrel will typically yield a greater muzzle velocity, while a short barrel will yield a lower one. In the case of the 5.56 NATO, M193 ammunition loses or gains approximately 25.7 feet-per-second for each inch of barrel length, while M855 loses or gains 30.3 feet-per-second per inch of barrel length.[31]

P.O. Ackley created an improved version of this cartridge, called the .223 Ackley Improved.[32] It has the straight sides and steep shoulder, typical of the Ackley design improvements, yielding about 5% extra case volume. This, in turn, provides longer case life, less stretching, and up to 140 ft/s (43 m/s) faster velocities.[33]

Wildcat cartridge developers have for a long time necked this cartridge up to create the 6mm/223 or 6×45mm. At one time this round was very popular for varminting and competition, but has been replaced by current popular competition cartridges using short, fat cases, such as the 6 mm PPC and the 6mm Norma BR.[citation needed]